Lithographic apparatus and device manufacturing method

ABSTRACT

A lithographic projection apparatus includes an alignment sensor having an electron beam source constructed and arranged to provide an electron beam for impinging on an alignment marker on a substrate, and a back-scattered electron detector constructed and arranged to detect electrons back-scattered from the alignment marker. The alignment sensor is independent of the projection system and projection radiation, and is an off-axis alignment sensor.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a lithographic projectionapparatus and a device manufacturing method.

[0003] 2. Description of the Related Art

[0004] The term “patterning device” as here employed should be broadlyinterpreted as referring to device that can be used to endow an incomingradiation beam with a patterned cross-section, corresponding to apattern that is to be created in a target portion of the substrate. Theterm “light valve” can also be used in this context. Generally, thepattern will correspond to a particular functional layer in a devicebeing created in the target portion, such as an integrated circuit orother device (see below). An example of such a patterning device is amask. The concept of a mask is well known in lithography, and itincludes mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. Placementof such a mask in the radiation beam causes selective transmission (inthe case of a transmissive mask) or reflection (in the case of areflective mask) of the radiation impinging on the mask, according tothe pattern on the mask. In the case of a mask, the support structurewill generally be a mask table, which ensures that the mask can be heldat a desired position in the incoming radiation beam, and that it can bemoved relative to the beam if so desired.

[0005] Another example of a patterning device is a programmable mirrorarray. One example of such an array is a matrix-addressable surfacehaving a viscoelastic control layer and a reflective surface. The basicprinciple behind such an apparatus is that, for example, addressed areasof the reflective surface reflect incident light as diffracted light,whereas unaddressed areas reflect incident light as undiffracted light.Using an appropriate filter, the undiffracted light can be filtered outof the reflected beam, leaving only the diffracted light behind. In thismanner, the beam becomes patterned according to the addressing patternof the matrix-addressable surface. An alternative embodiment of aprogrammable mirror array employs a matrix arrangement of tiny mirrors,each of which can be individually tilted about an axis by applying asuitable localized electric field, or by employing piezoelectricactuators. Once again, the mirrors are matrix-addressable, such thataddressed mirrors will reflect an incoming radiation beam in a differentdirection to unaddressed mirrors. In this manner, the reflected beam ispatterned according to the addressing pattern of the matrix-addressablemirrors. The required matrix addressing can be performed using suitableelectronics. In both of the situations described hereabove, thepatterning device can comprise one or more programmable mirror arrays.More information on mirror arrays as here referred to can be seen, forexample, from U.S. Pat. Nos. 5,296,891 and 5,523,193, and PCTpublications WO 98/38597 and WO 98/33096. In the case of a programmablemirror array, the support structure may be embodied as a frame or table,for example, which may be fixed or movable as required.

[0006] Another example of a patterning device is a programmable LCDarray. An example of such a construction is given in U.S. Pat. No.5,229,872. As above, the support structure in this case may be embodiedas a frame or table, for example, which may be fixed or movable asrequired.

[0007] For purposes of simplicity, the rest of this text may, at certainlocations, specifically direct itself to examples involving a mask andmask table. However, the general principles discussed in such instancesshould be seen in the broader context of the patterning device ashereabove set forth.

[0008] Lithographic projection apparatus can be used, for example, inthe manufacture of integrated circuits (ICs). In such a case, thepatterning device may generate a circuit pattern corresponding to anindividual layer of the IC, and this pattern can be imaged onto a targetportion (e.g. comprising one or more dies) on a substrate (siliconwafer) that has been coated with a layer of radiation-sensitive material(resist). In general, a single wafer will contain a whole network ofadjacent target portions that are successively irradiated via theprojection system, one at a time. In current apparatus, employingpatterning by a mask on a mask table, a distinction can be made betweentwo different types of machine. In one type of lithographic projectionapparatus, each target portion is irradiated by exposing the entire maskpattern onto the target portion at once. Such an apparatus is commonlyreferred to as a wafer stepper. In an alternative apparatus, commonlyreferred to as a step-and-scan apparatus, each target portion isirradiated by progressively scanning the mask pattern under theprojection beam in a given reference direction (the “scanning”direction) while synchronously scanning the substrate table parallel oranti-parallel to this direction. Since, in general, the projectionsystem will have a magnification factor M (generally<1), the speed V atwhich the substrate table is scanned will be a factor M times that atwhich the mask table is scanned. More information with regard tolithographic devices as here described can be seen, for example, fromU.S. Pat. No. 6,046,792.

[0009] In a known manufacturing process using a lithographic projectionapparatus, a pattern (e.g. in a mask) is imaged onto a substrate that isat least partially covered by a layer of radiation-sensitive material(resist). Prior to this imaging, the substrate may undergo variousprocedures, such as priming, resist coating and a soft bake. Afterexposure, the substrate may be subjected to other procedures, such as apost-exposure bake (PEB), development, a hard bake andmeasurement/inspection of the imaged features. This array of proceduresis used as a basis to pattern an individual layer of a device, e.g. anIC. Such a patterned layer may then undergo various processes such asetching, ion-implantation (doping), metallization, oxidation,chemo-mechanical polishing, etc., all intended to finish off anindividual layer. If several layers are required, then the wholeprocedure, or a variant thereof, will have to be repeated for each newlayer. It is important to ensure that the overlay juxtaposition) of thevarious stacked layers is as accurate as possible. For this purpose, asmall reference mark is provided at one or more positions on the wafer,thus defining the origin of a coordinate system on the wafer. Usingoptical and electronic devices in combination with the substrate holderpositioning device (referred to hereinafter as “alignment system”), thismark can then be relocated each time a new layer has to be juxtaposed onan existing layer, and can be used as an alignment reference.Eventually, an array of devices will be present on the substrate(wafer). These devices are then separated from one another by atechnique such as dicing or sawing, whence the individual devices can bemounted on a carrier, connected to pins, etc. Further informationregarding such processes can be obtained, for example, from the book“Microchip Fabrication: A Practical Guide to Semiconductor Processing”,Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN0-07-067250-4.

[0010] For the sake of simplicity, the projection system may hereinafterbe referred to as the “lens.” However, this term should be broadlyinterpreted as encompassing various types of projection system,including refractive optics, reflective optics, and catadioptricsystems, for example. The radiation system may also include componentsoperating according to any of these design types for directing, shapingor controlling the projection beam of radiation, and such components mayalso be referred to below, collectively or singularly, as a “lens”.Further, the lithographic apparatus may be of a type having two or moresubstrate tables (and/or two or more mask tables). In such “multiplestage” devices the additional tables may be used in parallel orpreparatory steps may be carried out on one or more tables while one ormore other tables are being used for exposures. Dual stage lithographicapparatus are described, for example, in U.S. Pat. No. 5,969,441 and WO98/40791.

[0011] Alignment is the process of positioning the image of a specificpoint on the mask to a specific point on the wafer that is to beexposed. For this to be achieved, the position and orientation of thewafer needs to be established, and for this purpose typically one ormore alignment markers, such as a small pattern, are provided on thesubstrate (wafer). A device may consist of many layers that are built upby successive exposures with intermediate processing steps. Before eachexposure, alignment is performed to minimize any positional errorbetween the new exposure and the previous ones, such error being termedoverlay error. However, some of the intermediate processing steps maydeposit material on top of the alignment markers, and they will at leastbe buried under a layer of energy-sensitive material (resist), which cancause the alignment markers to be obscured which can result in overlayerrors. These and other processing steps may also lead to undesiredshifts of the measured aligned position.

[0012] Some lithographic projection apparatus use electron beamradiation for perfonning the exposures, and the projection electron beamis used for alignment purposes. However this suffers from the problemthat the energy of the projection electron beam is not tailored to thealignment marker and the particular layers disposed above the markerbecause the beam energy is typically around 100 keV. This can result inproblems of damage to the features and/or unwanted exposure of theresist during the alignment procedure, and poor contrast in observingthe alignment marker.

SUMMARY OF THE INVENTION

[0013] It is an aspect of the present invention to alleviate, at leastpartially, the above problems.

[0014] This and other aspects are achieved according to the invention ina lithographic apparatus including a radiation system constructed andarranged to provide a projection beam of radiation; a support structureconstructed and arranged to support a patterning device, the patterningdevice constructed and arranged to pattern the projection beam accordingto a desired pattern; a substrate table constructed and arranged to holda substrate; a projection system constructed and arranged to project thepatterned beam onto a target portion of the substrate; and an alignmentsensor including an electron beam source constructed and arranged toprovide an electron beam for impinging on an alignment marker on asubstrate on the substrate table; and a back-scattered electron detectorconstructed and arranged to detect electrons back-scattered from thealignment marker, wherein the alignment sensor is independent of theradiation system of the lithographic projection apparatus but isprovided within the lithographic projection apparatus to enablealignment to be performed in situ within the lithographic projectionapparatus.

[0015] The present invention allows alignment to be performed using thealignment sensor even when the alignment marker is obscured or laterallyshifted by a layer of resist and/or further process layers deposited onthe substrate. The energy of the electron beam employed in the alignmentsensor can be tailored to the particular wafer, process layers andalignment marker independently of the projection radiation and of theradiation system and projection system.

[0016] Preferably the projection beam of radiation is EUV radiation.This allows the electron beam of the alignment sensor and the projectionradiation to be independent of each other. Moreover, both the projectionand the alignment system can operate in vacuum.

[0017] Preferably the electron beam and the substrate table are capableof being scanned relative to each other. This provides a detected signalas a function of scan position indicative of the position of thealignment marker. The electron beam may be scanned relative to thesubstrate table, or the substrate table may be scanned relative to theelectron beam.

[0018] The electron beam may be controllable to impinge on the substrateat a single spot. Alternatively, the electron beam may be controllableto impinge on the substrate at a predetermined intensity distributionhaving a pattern which corresponds, for example, with at least a portionof a pattern of the alignment marker on the substrate. Where this isdone, a mask may be provided to pattern the electron beam, the maskpreferably being provided with a pattern which is substantially anegative of the pattern of the alignment marker on the substrate.

[0019] Preferably the electron beam source is for providing electronswith an energy in the range of from 10 to 100 keV, and more preferablyin the range of from 20 to 50 keV. The electrons are thus sufficientlyenergetic to penetrate through any layers on top of the alignment markerbefore being back-scattered, but also provides good contrast inback-scattered coefficient between the substrate (e.g. silicon) and thehigher atomic number material of the marker (e.g. tungsten). If theelectron beam energy is too high, the contrast in back-scattering isreduced. In addition, for higher electron beam energies, the alignmentunit optical column must be longer, and is thus more difficult toincorporate into the lithographic apparatus.

[0020] According to a further aspect of the invention there is provideda device manufacturing method including providing a substrate that is atleast partially covered by a layer of radiation-sensitive material;providing a projection beam of radiation using a radiation system; usinga patterning device to endow the projection beam with a pattern in itscross-section; projecting the patterned beam of radiation onto a targetportion of the layer of radiation-sensitive material; and determiningthe position of the substrate prior to the projecting to align thesubstrate with respect to the patterned beam of radiation, by providingan electron beam independent of the projection beam; impinging theelectron beam on an alignment marker on the substrate; and detectingelectrons back-scattered from the alignment marker.

[0021] This further aspect of the invention may incorporate one or moreof the preferred features referred to above.

[0022] Although specific reference may be made in this text to the useof the apparatus according to the invention in the manufacture of ICs,it should be explicitly understood that such an apparatus has many otherpossible applications. For example, it may be employed in themanufacture of integrated optical systems, guidance and detectionpatterns for magnetic domain memories, liquid-crystal display panels,thin-film magnetic heads, etc. The skilled artisan will appreciate that,in the context of such alternative applications, any use of the terms“reticle”, “wafer” or “die” in this text should be considered as beingreplaced by the more general terms “mask”, “substrate” and “targetportion”, respectively.

[0023] In the present document, the terms “radiation” and “beam” areused to encompass all types of electromagnetic radiation, includingultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157 or126 nm) and EUV (extreme ultra-violet radiation, e.g. having awavelength in the range 5-20 nm), as well as particle beams, such as ionbeams or electron beams.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings inwhich:

[0025]FIG. 1 depicts a lithographic projection apparatus according to anembodiment of the present invention;

[0026]FIG. 2 is a schematic illustration of an alignment sensor, whichuses an electron beam, for use in the embodiment of FIG. 1;

[0027]FIG. 3 is a schematic cross-section of a wafer with an alignmentmarker on which an electron beam is incident according to one embodimentof the present invention;

[0028]FIG. 4 depicts the back-scattered electron detection signal as theelectron beam in the embodiment of FIG. 3 is scanned across thealignment marker;

[0029]FIG. 5 is a schematic cross-section of a wafer with an alignmentmarker on which a patterned electron beam is incident according toanother embodiment of the present invention; and

[0030]FIG. 6 depicts the back-scattered electron detection signal as theelectron beam in the embodiment of FIG. 5 is scanned across thealignment marker.

[0031] In the Figures, corresponding reference symbols indicatecorresponding parts.

DETAILED DESCRIPTION

[0032]FIG. 1 schematically depicts a lithographic projection apparatus 1according to an embodiment of the invention. The apparatus 1 includes abase plate BP; a radiation system including an illumination system(illuminator) IL and a source LA that is constructed and arranged tosupply a beam of radiation (e.g. an undulator or wiggler provided aroundthe path of an electron beam in a storage ring or synchrotron, a plasmasource, an electron or ion beam source, a mercury lamp or a laser). Thebeam is preferably an EUV beam, but may be any other suitable beam. Thebeam is caused to traverse various optical components included in theilluminator IL so that the resultant projection beam PB has a desiredshape and intensity distribution in its cross section.

[0033] The projection beam PB subsequently impinges upon a mask MA whichis held in a mask holder on a first object (mask) table MT that isconnected to a first positioning device PM that accurately positions themask with respect to a projection system or lens PL which focuses theprojection beam PB onto a target portion C of a substrate W2 or W3 heldon a second object (substrate) table W2T or a third object (substrate)table W3T, each provided with a substrate holder constructed andarranged to hold the substrate W2, W3 (e.g. a resist-coated siliconwafer), and connected to a second positioning device P2W or a thirdpositioning P3W that accurately positions the substrates W2, W3 withrespect to the lens PL. The lens PL (e.g. a mirror group) is constructedand arranged to image an irradiated portion of the mask MA onto a targetportion C (e.g. comprising one or more dies) of the substrates W2, W3.

[0034] With the aid of the second and third positioning devices P2W, P3Wand interferometers 1F, the substrate tables W2T, W3T can be movedaccurately, e.g. so as to position different target portions C in thepath of the beam PB. Similarly, the first positioning device PM can beused to accurately position the mask MA with respect to the path of thebeam PB, e.g. after mechanical retrieval of the mask MA from a masklibrary, or during a scan. In general, movement of the object tables MT,WT will be realized with the aid of a long-stroke module (coarsepositioning) and a short-stroke module (fine positioning), which are notexplicitly depicted in FIG. 1. However, in the case of a wafer stepper(as opposed to a step and scan apparatus) the mask table MT may just beconnected to a short stroke actuator, or may be fixed. The mask MA andthe substrate W may be aligned using mask alignment marks M₁, M₂ andsubstrate alignment marks P₁, P₂.

[0035] The depicted apparatus can be used in two different modes:

[0036] 1. In step mode, the mask table MT is kept essentiallystationary, and an entire mask image is projected at once, i.e. a single“flash,”onto a target portion C. The substrate table WT is then shiftedin the X and/or Y directions so that a different target portion C can beirradiated by the beam PB;

[0037] 2. In scan mode, essentially the same scenario applies, exceptthat a given target portion C is not exposed in a single “flash.”Instead, the mask table MT is movable in a given direction (theso-called “scan direction”, e.g., the Y direction) with a speed v, sothat the projection beam PB is caused to scan over a mask image.Concurrently, the substrate table WT is simultaneously moved in the sameor opposite direction at a speed V=Mv, in which M is the magnificationof the lens PL (typically, M=¼ or ⅕). In this manner, a relatively largetarget portion C can be exposed, without having to compromise onresolution.

[0038]FIG. 2 shows the principles of an alignment system AL embodyingthe present invention. The alignment system AL includes an electron beamsource 10 constructed and arranged to provide an electron beam 12directed towards a wafer W supported on a wafer table WT. When theelectron beam 12 impinges on a suitable alignment marker 14 on the waferW, back-scattered electrons are generated which are detected by detector16. Details of suitable electron beam sources 10 and back-scatteredelectron detectors 16 are known, for example, from the field of electronmicroscopy.

[0039] The back-scattering efficiency of the alignment marker 14 dependson the atomic number of the element or elements used to form the marker.It is preferred to use relatively high atomic number materials, forexample, tungsten, tantalum, cobalt or titanium, although othermaterials may also be employed for the marker, such as copper (thesilicides or nitrides of these metals may also be used). The marker mayconsist of multi-layered structures. The back-scattering efficiency, andits dependence on the electron beam position, also depend on the lateraland cross-sectional geometry of the marker. Specifically the depth,thickness, width and spatial separation of lines forming the marker, andmade of high atomic number material, are factors to be considered indetermining the back-scattering efficiency.

[0040] The precise position of the back-scattered electron detector 16is not critical because the back-scattering process is not particularlydirectional.

[0041] The electron beam 12 is scanned relative to the wafer W. Thescanning can be performed by moving the wafer table WT underneath astationary electron beam 12, or by deflecting the electron beam 12 usingelectric and/or magnetic fields, or by a combination of both.

[0042] The alignment system AL enables the position and orientation ofthe marker 14 on the wafer W, and hence the wafer W itself, to beaccurately determined with respect to the wafer table WT. Referring toFIG. 1, the alignment system AL is used to determine the position andorientation of the wafer W3 on the wafer table W3T. The wafer table W3Tis then transferred, via translation of the third positioning device P3Wto a location below the lens PL. This is the location occupied by thesecond positioning device P2W in FIG. 1 (the second positioning deviceP2W is translated substantially simultaneously to the location occupiedby the third positioning device P3W in FIG. 1). The third positioningdevice P3W is aligned with respect to the projection beam PB. Since theposition of the marker 14 (and the wafer W3) is known relative to thethird positioning device P3W, this allows correct alignment of the waferW3 with respect to the projection beam PB to allow an exposure to beperformed.

[0043] In an alternative embodiment of the invention (not shown) thealignment system AL may be located adjacent the lens PL, so thatalignment is achieved without the need to move the wafer and thepositioning device to an alternative, displaced, location.

[0044] In some instances it may be preferred to provide two alignmentsystems, a first located as shown in FIG. 1, and a second (not shown)located adjacent the lens PL.

[0045] Determination of the marker position/orientation will bediscussed in the context of further embodiments below.

[0046] The above described electron beam alignment sensor according tothis embodiment of the present invention is a so-called off-axisalignment sensor because the electron beam radiation does not passthrough the optical center of the lens PL. Indeed, in this embodiment ofthe present invention, the projection beam is EUV radiation and so hasoptics entirely independent of the electron beam alignment radiation.The electrons in the electron beam 12 generated by the electron beamsource 10 have an energy set in the range of from 10 to 100 keV, andusually between 20 and 50 keV, typically around 30 keV.

[0047] The embodiment of the present invention shown in FIG. 2 may beimplemented as shown in FIG. 3. Referring to FIG. 3, an electron beam 12is controlled to impinge on the wafer W at a single spot. In thisexample, the wafer W comprises a substrate 20 on which there is analignment marker 14. The marker 14 is in the form of a periodic gratingmade of stripes of tungsten. Only three of the stripes 14.1, 14.2 and14.3 are illustrated end-on in cross-section in FIG. 3. On top of thesubstrate 20 and the marker 14 is a layer 22 representing the resist andany process layers that have already been laid down on the wafer W. Theelectron beam 12 is scanned relative to the wafer W as indicated by thearrow 24 (though as mentioned above, the wafer W may alternatively bescanned in the opposite direction, or indeed both the wafer and theelectron beam 12 can be moved).

[0048] The resulting signal, related to the intensity of back-scatteredelectrons detected by the detector 16 as a function of scan position, isshown in FIG. 4. As can be seen, there is a correspondence between thepattern of the marker 14 and the signal in FIG. 4. Although the marker14 in FIG. 3 consists of a rectangular profile metal stripes 14. 1,14.2, 14.3, the actual signal detected by the detector 16 is theconvolution of the marker pattern and the electron beam profile, sothere is some smoothing in the detected signal of FIG. 4. The degree ofsmoothing depends on, for example, the width and profile of the electronbeam 12. A further broadening of the signal, i.e. the detectedback-scattered electron intensity as a function of electron beamposition, occurs due to electron scattering processes in the stack. Fromthe detected signal, the position and orientation of the marker 14 canbe determined. As with optical alignment markers, several gratings maybe provided, for example with different periods and for example atdifferent orientations, in order to uniquely determine the position andorientation of the marker and hence of the wafer.

[0049] In an alternative embodiment, shown in FIG. 5, the waferstructure of FIG. 3 is used, but the electron beam 12 is composed of aplurality of beamlets 12.1, 12.2, 12.3 and so on, such that the electronbeam 12 is patterned in a similar way to at least a portion of themarker 14. The electron beam 12 and wafer W are scanned relative to oneanother as described above and the resulting detected signal fromdetector 16 is shown in FIG. 6. At scan positions at which there iscorrespondence between the pattern of the electron beam 12 and themarker 14 there is a peak in the detected signal. Again the actualdetected signal is the convolution of the pattern of the marker 14 withthe intensity distribution of the electron beam 12. The same comments asthe embodiment above apply regarding using several gratings in themarker 14 to uniquely determine the position and orientation of thewafer W.

[0050] One way to obtain the patterned electron beam 12 according tothis embodiment is to pass an initial electron beam through as mask (notshown) which is essentially a negative of the desired portion of themarker 14. The features of the mask can be defined, for example, usingtungsten or other suitable material to block the electrons in particularportions of the pattern corresponding to the gaps between the featuresof the marker 14.

[0051] An advantage of using the electron beam to provide alignment, ascompared to optical beam alignment, is that the electron beam providesalignment with respect to alignment markers that cannot be seen via anoptical beam. Referring to FIG. 3, if layer 22 were to be opticallyopaque then it would not be possible to see the alignment markers usingan optical beam. The electron beam passes directly through the opticallyopaque layer 22, thereby allowing alignment to be achieved.

[0052] A further advantage of using the electron beam is that scatteringof the beam by the alignment markers is a function of the volume of thealignment markers not just the upper surface of the alignment markers(as is the case with optical alignment). This makes the alignment lesssensitive to asymmetry of the alignment markers, as compared withoptical alignment.

[0053] The invention is particularly suited to use in a lithographicapparatus which utilises EUV, since it operates well under vacuumconditions (vacuum is required for EUV to avoid absorption of the EUV bygas molecules).

[0054] While specific embodiments of the invention have been describedabove, it will be appreciated that the invention may be practicedotherwise than as described. The description is not intended to limitthe invention.

What is claimed is:
 1. A lithographic projection apparatus, comprising:a radiation system constructed and arranged to provide a projection beamof radiation; a support structure constructed and arranged to support apatterning device, the patterning device constructed and arranged topattern the projection beam according to a desired pattern; a substratetable constructed and arranged to hold a substrate; a projection systemconstructed and arranged to project the patterned beam onto a targetportion of the substrate; and an alignment sensor comprising: anelectron beam source constructed and arranged to provide an electronbeam for impinging on an alignment marker on a substrate on thesubstrate table; and a back-scattered electron detector constructed andarranged to detect electrons back-scattered from the alignment marker,wherein the alignment sensor is independent of the radiation system ofthe lithographic projection apparatus but is provided within thelithographic projection apparatus to enable alignment to be performed insitu within the lithographic projection apparatus.
 2. An apparatusaccording to claim 1, wherein the projection beam of radiation is EUVradiation.
 3. An apparatus according to claim 1, wherein the electronbeam and the substrate table are scannable relative to each other.
 4. Anapparatus according to claim 3, wherein the substrate table is fixed,and the electron beam is scannable relative to the substrate table. 5.An apparatus according to claim 3, wherein the electron beam is fixed,and the substrate table is scannable relative to the electron beam. 6.An apparatus according to claim 1, wherein the electron beam iscontrollable to impinge on the substrate at a single spot.
 7. Anapparatus according to claim 1, wherein the electron beam is patternableto impinge on the substrate at a predetermined intensity distribution.8. An apparatus according to claim 7, wherein the intensity distributionis predetermined to correspond with at least a portion of a pattern ofthe alignment marker on the substrate.
 9. An apparatus according toclaim 8, further comprising a mask constructed and arranged to patternthe electron beam.
 10. An apparatus according to claim 9, wherein themask is provided with a pattern which is substantially a negative of thepattern of the alignment marker on the substrate.
 11. An apparatusaccording to claim 1, wherein the electron beam source is constructedand arranged to provide electrons with an energy in the range of 10 to100 keV
 12. An apparatus according to claim 1, wherein the electron beamsource is constructed and arranged to provide electrons with an energyin the range of 20 to 50 keV.
 13. An apparatus according to claim 1,wherein the support structure comprises a mask table constructed andarranged to hold a mask.
 14. An apparatus according to claim 1, whereinthe radiation system comprises a radiation source.
 15. A devicemanufacturing method, comprising: providing a substrate that is at leastpartially covered by a layer of radiation-sensitive material; providinga projection beam of radiation using a radiation system; using apatterning device to endow the projection beam with a pattern in itscross-section; projecting the patterned beam of radiation onto a targetportion of the layer of radiation-sensitive material; and determiningthe position of the substrate prior to the projecting to align thesubstrate with respect to the patterned beam of radiation, by: providingan electron beam independent of the projection beam; impinging theelectron beam on an alignment marker on the substrate; and detectingelectrons back-scattered from the alignment marker.
 16. A methodaccording to claim 15, wherein the projection beam of radiation is EUVradiation.
 17. A method according to claim 15, wherein the electron beamand the substrate table are scanned relative to each other.
 18. A methodaccording to claim 17, wherein the substrate table is fixed and theelectron beam is scanned relative to the substrate table.
 19. A methodaccording to claim 17, wherein the electron beam is fixed and thesubstrate table is scanned relative to the electron beam.
 20. A methodaccording to claim 15, wherein the electron beam is controlled toimpinge on the substrate at a single spot.
 21. A method according to anyclaim 15, wherein the electron beam is patterned to impinge on thesubstrate at a predetermined intensity distribution.
 22. A methodaccording to claim 21, wherein the intensity distribution ispredetermined to correspond with at least a portion of a pattern of thealignment marker on the substrate.
 23. A method according to claim 22,wherein a mask is used to pattern the electron beam.
 24. A methodaccording to claim 23, wherein the mask is provided with a pattern whichis substantially a negative of the pattern of the alignment marker onthe substrate.
 25. A method according to claim 15, wherein electrons ofthe electron beam have an energy in the range of 10 to 100 keV.
 26. Amethod according to claim 15, wherein the electrons of the electron beamhave an energy in the range of 20 to 50 keV.